I recommend plumbing an aux tank in before the regulator.. Now, that said I do know there are SOME rigs where this is not an easy task due to the placement of said regulator.
But on most rigs you simply go to your RV store and pick up one of two things: Most stores the Marshall brass Extend-a-Stay, (I will link to the catalog page if I can) the At camping world it's called "Stay-a-while" Basically same product.
This is the ONLY way I recommend doing it, they have several safety features that you won't get by doing a home brew.. Oh, I know you will never forget to turn that valve.. But what happens if someone else does the hook up.. Propane explosions are nasty.
The Marshall Brass web page is being re-done so here is a retailler's page.. Note, THis is not a comment on the retallier, Only found 'em with a Google search:
https://www.makariosrv.com/products/...all-Brass.html
Note the one pictured may not fit your rig, there are other versions, the one I have is straight through and POL fittings on both "Main" inlet and on outlet, You can Mix and match so you get POL on the RV's tank and regulator and Acme on the protable if you liek.
The one I have looks exactly like the camping world "Stay-a-while" linked below from Mr. Heater.
Mr Heater Corp - Stay A While Tee - Propane Accessories - Camping World
The delux kit includes hoses to hook to the portable tanks.
While I THINK this is the same product as Marshall Brass makes, I do not know for sure, THe M.B. unit has check valves to insure no gas flowing tank to tank and other than what's in the hose, no lost Propane when you disconnect.
Both of them have a Quick-Disconnect (like a throw away bottle) for hooking up a 10' hose to the Grill.. The M.B. unit I have came with that hose as well plus I have a 2nd 10 feet, again, you loose the gas in the hose when you unhook,
These all work on the high pressure side
If you try to feed in gas via a low pressure feed, you need a cut off vavle between teh feed point and the regulator or the regulator will vent gas on you,, Not a good thing.